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You can disable the context menu, which is where users typically access the full screen mode. Here's more information on our DivX Developer Portal:
http://developer.divx.com/docs/divx_web_player/embed_parameters/allowContextMenu
I don't think that's possible but I could be wrong. The fullscreen is part of a function of all media players. To disable that is to cripple what a video player should be. So it is kinda ridiculous to go backward. Don't use Flash as an example because that would not be a video player by nature.